This invention relates to a method for converting colors from a first device color space to gray levels for reproduction by a color or monochrome rendering device which employs a single color marking media to achieve the gray levels. More particularly, the invention relates to a method for attaining improved gray representations from a printer in response to a color image input from a monitor.
It is well known that color monitors exhibit a larger color gamut than currently available laser and inkjet color printers. The prior art has suggested a number of methods for achieving more faithful reproduction of such monitor colors from computer printers. For instance, U.S Pat. No. 5,081,529 to Collette describes a color calibration system that enables a user to achieve such more faithful color reproduction. Essentially, a hard copy print is made from a processed image and if it does not match the monitor image colors, a color correction algorithm is invoked which is responsive to operator adjustments to revise the color controls within the printer. Accordingly, after several iterations, a more faithful color reproduction from the printer is achieved.
To convert monitor red, green and blue (RGB) values to gray levels for reproduction by a color printer, the prior art has converted the monitor RGB color values to CIE L*a*b* color values (via an intermediate XYZ tristimulus color space conversion). Thereafter, the lightness (i.e., L*) values have been directly mapped to printer gray levels. Once such a gamut mapping was achieved, the resulting mapped values were converted to a table which was used to control the printer.
A more common prior art procedure for converting RGB colors to gray levels is described in SMPTE RECOMMENDED PRACTICE, RP 177-1993, section 3.3.8. In that document, the luminance value (i.e., the Y coordinate in XYZ tristimulus color space) is defined as:
Y=YR(R)+YG(G)+YB(B)
The derived Y luminance values are then scaled to device gray levels.
Gamut mapping from a monitor""s color space to a color printer""s gray levels has presented certain special problems. For instance, the above indicated color conversion procedure results in certain color components not reproducing well in the resulting gray image. For example, certain cyan components and yellow components of the image will disappear. This is especially the case when reproduction of light yellow and cyan monitor colors are attenuated by the printerxe2x80x94as the printer is particularly adapted to producing dark greens and not the lighter shades. Therefore, the lighter shades fail to be reproduced.
Accordingly, it is an object of this invention to provide an improved method for converting colors from a first device color space to gray levels which can be reproduced by a single color marking media in a color rendering device.
It is a further object of this invention to provide a method for converting colors from a first device color space to gray levels in a second device color space wherein improved gray representations for certain colors containing either green or blue are achieved.
The method of the invention converts colors resident in a first device color space to gray levels for reproduction by a single color marking media in a color or monochrome rendering device. The method converts a set of red, green and blue (RGB) primary color values received from a processor to gray level values. The method applies a gamma correction function to the RGB primary color values and further scales the values to a range of from 0 to 1. The method then converts the gamma-corrected and scaled RGB primary color values to a Y luminance value (in the XYZ tristimulus color space) through use of the expression:
Y=YR(R)+(YGxe2x88x92kG)(G)+(YB+kB)(B)
where: YR, YG, and YB are coefficients that provide a luminance value in the XYZ color space in accordance with accepted observer standards; kG is a correction value that renders the green component darker; and kB also renders the blue component lighter. The resulting luminance value is then converted to a device gray value and stored in a table for later use.